Temperature Conversion from Fahrenheit to Celsius and back
Mental Math Rule Derived from Slide Rule
With the global hot summer months dwelling, it is sometimes useful to compare the daily temperatures of big cities around the world.
In most places, local residents are used to temperature units in Celsius, except US and a few places where Fahrenheit is still in use. (https://worldpopulationreview.com/country-rankings/countries-that-use-fahrenheit)